class Solution:
    def numTrees(self, n):
        """
        :type n: int
        :rtype: int
        """
        dp = [0 for _ in range(n + 1)]
        dp[0] = dp[1] = 1
        for i in range(2, n + 1):
            for j in range(1, n + 1):
                dp[i] += dp[j - 1] * dp[i - j]
        return dp[n]
